The invention concerns a tubular filter element or cartridge for liquid filter devices. Such a filter element typically comprises a perforated support pipe. A filter layer comprising a folded sheet of filter material is applied against an inside periphery of the pipe. A pair of disks are located on both ends of the filter element, the disks being joined to the support pipe and the filter layer by a hardened cast-in material.
During use of the filter, the liquid to be filtered flows through the tubular filter element from the inside out, i.e., a pressure builds up inside the filter element that is higher than the pressure on the outside of the filter element. The differential pressure established in this manner on the filter element rises with an increasing degree of clogging of the filter. Although in the actual operation of conventional filter elements, pressure differences on the order of only 1.1 bar (15 psi) occur, during testing of the filter element, very high pressure differences of up to 5.2 bar (75 psi) are specified, whereby the filter elements are highly stressed.
Particularly high requirements are set for certain liquid filter elements which are utilized in high pressure environments. In the case of a known filter element (see German Publication DE-21 26 080) several layers of fiber material are applied to the outside of a perforated support pipe, in particular for coalescence. A sheet of filter material, folded and arranged in a tubular shape, is applied to the inside of the pipe. The perforated support pipe and all of the filter and separating layers of these known filter elements are adhesively bonded at their ends to annular disks, provided with internal and external beaded rims. Usually, the ends of the filter material are cast within the disks by means of a hardenable molding material, in particular, a plastic resin which hardens after being poured into the disk, thereby establishing an intimate connection or adhesive bond of the disk with the ends of the filter element.
Due to the pressure difference existing between the inner and outer sides of the filter element, the tubular folded filter layer is deformed into contact with the inside of the perforated support pipe and is thus supported by the pipe, so that even relatively high difference pressures cannot damage or destroy the filter layer. However, such a radial support of the filter layer against the support pipe occurs only if the filter layer is able to bear firmly against the support pipe under the effect of the pressure differential. But movement or deformation of the filter layer into contact with the pipe cannot occur at the ends of the filter layer, since those ends are resin-cast or otherwise fixedly joined to the disks.
Since the ends of the folded filter layer are unable to bear against the support pipe in such a manner as to be suitably supported for resisting the effect of the internal pressure, the folded filter layer becomes stressed in shear at its ends. This stress, which could lead rapidly to the damaging or destruction of the folded filter layer, is particularly high if, during the resin-casting operation, a gap is created between the outer circumference of the folded filter layer and the inside of the support pipe.
It is, therefore, an object of the invention to provide a filter element of the afore-mentioned type in filter layer at the resin-cast ends may be prevented safely with simple structural means.